Exfoliated graphite can be obtained by first intercalating natural graphite with an intercalating agent to form a graphite intercalation compound that is then exposed to a thermal shock, for example, at a temperature of 700° C.-1,050° C. for a short duration of time (20-60 seconds) to expand or exfoliate the graphite. Exfoliated graphite particles are vermiform in appearance, and are commonly referred to as “worms”. The worm is a network of interconnected, thin graphite flakes, with pores present between flakes that make the worms compressible. FIG. 1 is a scanning electron microscope image illustrating the structure of exfoliated graphite.
Exfoliated graphite can be compressed to form a low density mat, or to form sheets of higher density material, referred to as “flexible graphite” or “exfoliated graphite sheet” or “graphite sheet”. A calendering process, where exfoliated graphite material is fed through a series of drums or rollers in a process that gradually brings the material to a desired thickness and density range, can be used for forming flexible graphite. Flexible graphite can be mechanically processed, formed and/or cut into various shapes, and generally can be wound up on a drum to form a roll. A typical process for the preparation of flexible graphite from expanded or exfoliated graphite particles is described in U.S. Pat. No. 3,404,061. Flexible graphite typically has a density in the range of about 0.2 g/cm3 to about 1.9 g/cm3, and is commonly available at densities in the range of 0.7 g/cm3 to 1.4 g/cm3 (the theoretical maximum density being 2.26 g/cm3). Calendering or compression forming steps can also be used to emboss features on one or both surfaces of flexible graphite sheet material that are suitable for its end-use application.
FIGS. 2A and 2B are scanning electron microscope images illustrating exfoliated graphite sheet materials (flexible graphite) of a lower density structure and a higher density structure, respectively. The lower density structure of FIG. 2A has a density of approximately 0.46 g/cm3. The higher density structure of FIG. 2B has a density of approximately 1.02 g/cm3.
Most of the graphite flakes in flexible graphite are oriented parallel to the two opposed major exterior surfaces. Although flexible graphite is typically highly electrically conductive (typically around 1,300 S/cm) in the in-plane directions, the through-plane electrical conductivity of flexible is significantly less (often only about 15 S/cm). The anisotropy ratio, the ratio of highest electrical conductivity to lowest conductivity values, is typically as high as 86:1 (and often higher than this value). The thermal properties of conventional flexible graphite are similarly highly anisotropic with the in-plane thermal conductivity being many times greater than the through-plane conductivity.
The properties of flexible graphite can be adjusted by incorporating a resin during forming of the material or impregnating it with a resin or another suitable impregnation medium after it is formed. The impregnation medium at least partially fills the pores between the graphite flakes. Resins suitable for impregnation of flexible graphite include phenolic, furan, epoxy and acrylic resins. Other additives are sometimes incorporated into flexible graphite.
For thermal management applications, such as heat sinks, heat spreaders and thermal interfaces, flexible graphite offers many advantages over other materials that are commonly used in these applications such as copper, steel and aluminum. For example, relative to these metals, flexible graphite is generally lighter, less susceptible to corrosion, has lower thermal expansion and has higher thermal conductivity in the in-plane direction.